The present invention relates to a method for manufacturing a solid state imaging device and, in particular, to a method for manufacturing a photodiode layer and a shielding layer formed on the photodiode layer in a solid state imaging device.
In a photoelectric conversion element formed in the solid state imaging device, for example, a P-type impurity diffusion region is formed at a surface region of an N-type semiconductor substrate and a PN junction is formed at a boundary between these regions. The PN junction functions as a light receiving element which performs photoelectric conversion. On the P-type impurity diffusion region, an N+-type impurity diffusion region is formed. The N+-type impurity diffusion region functions as a shielding layer for protecting the P-type impurity diffusion region from incorporation of any other substance or from thermal energy in a manufacturing process. On the N-type semiconductor substrate formed with the P-type impurity diffusion region and the N+-type impurity diffusion region, an oxide film serving as an insulating film is formed on the whole surface thereof. On the oxide film, there is provided a gate electrode for transferring a signal charge produced by the photoelectric conversion element to a signal charge transfer section.
In forming the photoelectric conversion element for the solid state imaging device, conventionally, a shielding layer is formed by injecting an ion to be an impurity with high ion injection energy at a time. However, an energy range used for ion injection in forming the shielding layer, which is an impurity diffusion region, is much larger than the binding energy of atoms forming a crystal of a semiconductor substrate and therefore the atoms brought into collision with an incident ion are plastically deformed and are cut off from lattice points, thus generating crystal defect. The crystal defect caused by the plastic deformation weakens binding energy between lattices of semiconductor atoms and an energy level is formed in the energy level within a forbidden band where no crystal failure exists. Accordingly, a band gap becomes narrow and electrons move from a valence electron band to a conductive band in the forbidden band, thereby causing a dark current.
Generally, there has been used a method for recovering the crystal defect by annealing a semiconductor substrate which has undergone ion injection, however, a heavy metal trap during this process makes an energy level in the forbidden band, which causes a dark current.
For a solution of such a problem with ion injection process, there has been known a method for preventing crystal defect from generating even when high energy ion injection is performed.
Specifically, Japanese Patent Application Laid-Open No. 2007-109818 has disclosed that in forming an impurity diffusion layer, ion injection to the vicinity of a semiconductor substrate surface is performed with injection energy of at least 100 keV and at most 300 keV, using an ion having approximately same atomic radius as that of an element used for the substrate and, on the contrary, ion injection to a deep section of the semiconductor substrate is performed with injection energy of at least 300 keV and at most 4,000 keV, using an ion having approximately same mass as that of an element used for the substrate.
However, such a process requires use of different types of ions for forming the same impurity diffusion region, which causes a problem of complexity in a manufacturing process.
As described above, in a conventional process for forming an impurity diffusion layer, crystal defect occurs because ion injection is performed with high energy and hence dark current is generated. As means for solving the foregoing problems, there is a method for forming the same impurity diffusion region using different ions, however, this method has a problem of complexity in manufacturing process.